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Radio source techniques are a common means of measuring the G/T of user antennas. These techniques depend on measuring noise parameters and are invalid when interference is present. The error sources in G/T measurements are described. The measurement and error sources of system noise temperature values, T, are also discussed and are needed when antenna gain values are required or when antenna gain levels are determined by other means and G/T is required.
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We present a systematic design procedure to evaluate diversity antenna systems. This method is unique in allowing the practical design for the minimum number of antennas to realize diversity systems satisfying a given communication performance. The proposed method is adopted herewith to design a body-worn antenna diversity system in a complex surrounding environment. To validate the method, a 4-channel diversity module is fabricated and the channel capacity of the entire diversity system is calculated and measured in a realistic scenario.
The rise in the number of active antennas used in radar applications calls for changes to the common absorber treatment used in chambers. The electronic circuits that are imbedded into these scanning arrays are non-linear in nature so they must be tested at the correct power outputs to get the correct pattern behavior. The combination of higher power and narrow beams means that areas of the anechoic treatment in a chamber can be subjected to high power densities. High power absorber has been used in the industry for many years. The substrate used in these absorbers makes the material very expensive. While in the past it was common to use this material only in regions where the main beam was going to be illuminating the absorber treatment the new electronically scanned arrays will have main beams that can illuminate several areas of the chamber. In cases where Near Field systems are used the absorber material will be in a radiation region where the main beam has not been formed, but the Absorber is closer to an array that is radiating high power so a large area of higher power absorber is needed to treat the chamber. In the present paper the authors present a medium power absorber 3kW per square meter (versus 775w per square meter) using the same material used in common RF absorber. Mechanical changes to the absorber are performed to increase the thermal dissipation of the EM energy absorbed. A series of measurement of the absorber is performed with a without additional air flow for cooling. The result is an absorber that can handle higher power densities without the need for exotic substrates.
This is the second of a two-part paper discussing the NASA Debris Radar (NDR) system developed to characterize debris liberated by the space shuttle during its ascent into space. While initial NDR missions proved the extent of the debris detection and tracking challenge, improvements in NDR hardware, software, and mission operations resulted in very successful debris detection and tracking. These successes lead to a new challenge of processing and analyzing the large amount of radar data collected by the NDR systems and extracting information useful to the NASA debris community. Analysis tools and software codes were developed to visualize the shuttle metric data in real-time, visualize metric and signature data during post-mission analysis, automatically detect and characterize debris tracks in signature data, determine ballistic numbers for detected debris objects, and assess material type, size, release location and threat to the orbiter based on radar scattering and ballistic properties of the debris.
During the STS-107 accident investigation, radar data collected during ascent indicated a debris event that was initially theorized to be the root cause of the accident. This theory was investigated and subsequently disproved by the Columbia Accident Investigation Board (CAIB). However, the data itself and the lack of understanding of what debris data in radar meant to the shuttle program, required further analysis and understanding. The Space Shuttle Program Systems Engineering and Integration (SE&I) Office commissioned the Ascent Debris Radar Working Group (ADRWG) to characterize the debris environment during a Space Shuttle launch and to identify/define the return signals as seen by radar. Once the capabilities and limitations of the existing radars for debris tracking were understood, the team researched proposed upgrades to the location, characteristics, and post-processing techniques needed to provide improved radar imaging of Shuttle debris. The research phase involved in assessing the threat ultimately evolved into an inter-agency cooperation between NASA and the Navy for shared use of radar assets to the benefit of both agencies. Additional cooperative agreements were made with the Air Force and Army regarding various support aspects to the debris radar efforts. An aggressive schedule of field testing preceded the initial operations of the system during the STS-114 Return to Flight (RTF) mission in July of 2005.
In a recent paper [1], we have introduced the concept of the time-reversal electromagnetic chamber (TREC), a new facility for creating coherent wave-fronts within a reverberation chamber. This facility, based on the use of time-reversal techniques in a reverberating environment, is here shown to be also a useful tool for the characterization of the field radiated by an antenna under test (AUT). The TREC is proven to be capable of providing real-time measurements, with an accuracy comparable to that of spherical near-field facilities, while using a very limited number of static probe antennas. This performance is made possible by taking advantage of the reflections over the chamber’s walls, in order to gain access to the field radiated along all the directions, with no need to mechanically displace the probes, or to have a full range of electronically switched ones. A 2D numerical validation supports this approach, proving that the proposed procedure allows the retrieval of the free-space radiation pattern of the AUT, with an accuracy below 1 dB over its main lobes.
The radiation characteristics of antennas can be deter-mined by measuring amplitude and phase data in the ra-diating near-field followed by a transformation to the far-field. Accurate phase measurements especially at high frequencies are very demanding in terms of the required measurement equipment and tolerances. Phaseless mea-surement techniques have been proposed, which often deal with a second set of amplitude only measurement data in order to compensate the lack of phase information. In this paper the concept of phaseless spherical near-field measurements will be addressed by presenting a phaseless near-field transformation algorithm for spherical antenna measurements, working with amplitude only data on two spheres. In particular the measurement of a patch antenna is considered to demonstrate the utility of the technique for low gain antennas. To address the issue of robustness, inaccurate measurement distances as well as spherical rotation angles are considered in order to evaluate the accuracy of the method against probe positioning errors. Furthermore noise contributions are introduced to emu-late measurement inaccuracies in general.
This paper presents a new approach to the inversion of boundary value (BV) problems in an infinite conductive, homogeneous media. Our interest is to investigate the possibility of imaging underwater electromagnetic sources from remote electromagnetic sensor data. Specifically, given two polarizations of the electric/magnetic fields on a cylindrical surface exterior to the electric and magnetic sources, we develop a frequency domain, back-projection technique that allows for the complete determination of the electric and magnetic fields in the region between the BV surface and the sources. This is an inverse problem and Tikhonov regularization is used to obtain an accurate filtered, back-propagated solution. In this approach two components of the measured field yield the six components of the field closer to the source. Of particular interest is the active part of the Poynting vector that is constructed from the back-projected fields, providing the power per unit area radiated from the sources. We believe it may be of immense practical use in diagnosis of electromagnetic sources underwater. We test the theory with a numerical experiment using a linear array of either magnetic or electric dipole sources excited in a frequency range of 1 to 1000 Hz in seawater that generate two cylindrical holograms (30m radius) of the axial polarization of the magnetic and electric fields, respectively. The complete (all polarizations) electric and magnetic fields are predicted along with the real and imaginary parts of the Poynting vector on a cylindrical back-propagation surface (20m radius). These simulations show that very accurate results are obtained even with low signal-to-noise levels. Work supported by the Office of Naval Research.
Polarimetric radar cross section systems are characterized by polarimetric system parameters Eh and Ev. These parameters can be measured with the use of rotating dihedrals. The full polarimetric dataset as a function of the angle of rotation can be analyzed with a nonlinear set of calibration equations to yield the system-parameter complex constants and the four polarimetric calibration amplitudes. These amplitudes appropriately reproduce the system drift and satisfy a drift-free system con.guration criterion very accurately. The results indicate that the nonlinear approach is better than the previously studied linear approach, which yielded system parameters that are seriously distorted by drift.
In previous papers [1]-[4] we have presented formulations for the circular and linear near field-to-far field RCS transformations (CNFFFT and LNFFFT, respectively). These formulations assumed that the target did not have significant extent above or below a central (waterline) plane, and that the circular or linear near field scans lied in this waterline plane. In this paper, the CNFFFT and LNFFFT formulations are generalized to scans that lie in a plane parallel to and above or below the waterline plane. These scans correspond to conical or great circle RCS cuts, respectively, in the far field at elevation angles other than 90°. We will show that the generalization can be accomplished by modifying just the frequency domain processing steps that are common to both algorithms, while leaving the spatial processing portions (apart from a minor variable redefinition) unchanged. The paper focuses on the mathematical derivation and numerical implementation of the algorithms; examples of numerical and experiment results are deferred to future papers.
Reflections in antenna test ranges can often be the largest source of measurement error within the error budget of a given facility [1]. Previously, a technique named Mathematical Absorber Reflection Suppression (MARS) has been used with considerable success in reducing range multi-path effects in spherical near-field antenna measurements [2, 3, 4, 5]. Whilst the technique presented herein is also a general purpose measurement and post-processing technique; uniquely, this technique is applicable to cylindrical near-field antenna test ranges. Here, the post-processing involves the analysis of the cylindrical mode spectrum of the measured field data which is then combined with a filtering process to suppress undesirable scattered signals.
Inverse equivalent current method has recently gained popularity in the applications of near-field far-field (NFFF) transformations especially when near-field (NF) measurements are carried out on irregular measurement grids around the arbitrarily shaped object under test. Usually low order (LO) Rao-Wilton-Glisson (RWG) basis functions or even point based low order basis functions are used for the discretization of the unknown surface current densities on the triangular discretization elements. Better accuracies are achievable when equal number of higher order (HO) basis functions is employed to represent unknown surface current densities. Nearly-orthogonal hierarchical vector basis functions complete to full first order with respect to the curl space are therefore utilized for the discretization of inverse equivalent surface currents defined on flat triangular domains. Various numerical examples are presented and comparison is made with the results of LO discretization.
Accurate calibration of near-field measurements requires the probe used for the measurement be well characterized. The determination of the absolute gain of rectangular open-ended waveguide probes is difficult due to the broad beamwidth in both the E-plane and H-plane which increase the likelihood of multi-path affecting the accuracy of the measurement. Multi-path may be minimized by reducing the separation distance, but at the price that far-field conditions may no longer apply. A variation of the two matched antenna method is to use a large reflecting plate to form an image of the probe. Use of the entire bandwidth of the probe, and time-gating the results to isolate the signal reflected from the plate allows the gain to be determined. The procedure also allows the determination of the aperture reflection coefficient used by theoretical probe models used for pattern compensation in the near-to-far-field transformation.
This paper examines the inter-relatedness between the polarization vector of the linearly polarized feed/probe, the analysis coordinate system used for the DL-to-CP transformation, and how the test antenna generates its circular polarization response. The measurement of the performance characteristics of circularly polarized antennas is often accomplished using a linearly polarized feed/probe to measure horizontal and vertical polarization components. The measured orthogonal linearly polarized components are combined using a mathematical technique to transform them to the orthogonal right-hand and left-hand circular polarization components of the test antenna. One of the difficulties in using this technique is insuring the proper orientation of the feed/probe for the measurement, the analysis coordinate systems used for the DL-to-CP transformation algorithm. Another is understanding the manner in which the circular polarization is being generated by the antenna under test. These factors are inherently related, and without proper care the wrong answer can easily be calculated.
The back wall is an important element in a high performance tapered or compact range anechoic chamber operating at VHF/UHF frequencies, as by design it is intended to absorb the non-intercepted portion of the incident plane wave containing the majority of the power transmitted by the chamber illuminator. Back wall reflections may interfere with the direct illumination signal and thus influence the test zone performance. Consequently, in order to ensure that the overall test zone reflectivity specification is met, the reflectivity produced by the back wall should be better than the reflectivity specified for the test zone. The conventional approach used to achieve good reflectivity is to apply high performance, high quality absorbing materials to the back wall. Further improvement of up to 10 dB can be achieved if a Chebyshev absorber layout is implemented [1, 2]. This layout consists of high performance absorbing pyramids of different heights, and assumes that the performance does not depend on a metallic backing plate. This approach is expensive, and presents technical challenges due to the complexity involved in the design and manufacturing of the absorbing material. In addition, installation and maintenance is an issue for such large absorbers. In this paper an alternative approach is presented which is based on an implementation of a shaped back wall as, for example, suggested in [3-5], and use of lighter, lower grade absorbing materials whose performance essentially depends on reflections from the metallic backing wall. This type of design can be optimized at the lowest operating frequency, if the back wall and absorber front face reflections cancel each other. Different back wall shapes are considered for a tapered chamber configuration, and the test zone reflectivity produced by a flat, inverted “open book” and a pyramidal back wall are evaluated and compared at VHF frequencies using a 3D EM transient solver [6].
During lightning strikes buildings and other structures can act as imperfect Faraday Cages, enabling electromagnetic fields to be developed inside the facilities. Some equipment stored inside these facilities may unfortunately act as antenna systems. It is important to have techniques developed to analyze how much voltage, current, or energy dissipation may be developed over valuable components. In this discussion we will demonstrate the modeling techniques used to accurately analyze a generic missile type weapons system as it goes through different stages of assembly. As work is performed on weapons systems detonator cables can become exposed. These cables will form different monopole and loop type antenna systems that must be analyzed to determine the voltages developed over the detonator regions. Due to the low frequencies of lightning pulses, a lumped element circuit model can be developed to help analyze the different antenna configurations. We will show an example of how numerical modeling can be used to develop the lumped element circuit models used to calculate voltage, current, or energy dissipated over the detonator region of a generic missile type weapons system.
The primary purpose of a chamber for Far–Field (FF) antenna measurements is to create a test zone surrounding the AUT, where the electric field is to be as uniform as possible, and multiple reflections are kept to a minimum. It is well known, that typical rectangular anechoic chambers for Far–Field (FF) antenna measurements are subject to increased reflectivity from specular regions on the side walls, floor and ceiling. The reflectivity further increases if a larger test zone and, consequently, longer source antenna/ AUT separation is required. The alternative to a rectangular chamber, which can be implemented to reduce the reflectivity, could be a chamber with a shaped interior, where the side walls are to be shaped based on GTD/GO principles so that the reflections are diverted out of the test zone. Even more reflectivity suppression is expected, if, in addition, wedge absorbers are used throughout the specular region or entire wall with a smoothly varied wedge orientation chosen according to GTD principles. The combination of two approaches constitutes a chamber design method termed a “Two – Level GTD”. The chamber shape and wedge orientation for delivering reduced reflectivity in the test zone are not unique. According to a “Two -Level GTD” a plurality of solutions exists and can be practically implemented. Freedom in choosing these parameters can be utilized to satisfy the additional requirements for the chamber design to reduce RCS clutter and/or secondary reflections in the chamber. In this paper the method validity is confirmed based on comparison of various chamber designs performed using 3D EM analysis tools.
ESA’s Compact Antenna Test Range at ESTEC has been relocated which has given the chance to improve the alignment of the reflectors. Based on measure-ments of the reflector surfaces the best-fit positions and orientations of the reflectors have been deter-mined. It turned out that the choice of parameters to describe the reflectors and their position had impor-tant impact on the optimization process: The parame-ters shall – as far as possible – be orthogonal in the sense that a change in one parameter must not influ-ence the final value of the other parameters.
Transmitting equipment may interfere with sensitive electronic equipment even if both are in compliance with regulatory standards. This paper examines the potential for electromagnetic interference (EMI) between Federal Communications Commission (FCC) Part 15.247 for Ultra-High Frequency (UHF) emitters and immunity compliant sensitive equipment. At close ranges, the electromagnetic (EM) fields from these UHF emitters may exceed minimum standard immunity testing levels. This does not imply that interference will occur, but that the device may not be qualified to operate in the EM environment near the emitter. UHF Radio Frequency Identification (RFID) emitters are deployed regularly in locations where sensitive electronic equipment is in use. Recent studies have indicated that an interference potential can exist between some UHF emitters and medical, commercial and military systems. This paper estimates the range at which FCC compliant devices may pose a risk to industrial, consumer and medical devices and compares it to previously published data.